1. Field of the Invention
The present invention relates to a method and apparatus for preheating fuel, and more particularly to such a method and apparatus for use with a diesel internal combustion engine to prevent fuel system blockage in cold weather due to crystallization in, or separation of, wax and the like from the fuel.
2. Description of the Prior Art
When the temperature of diesel fuel becomes sufficiently low, certain components of the fuel solidify into wax. This wax then blocks elements of a fuel system through which the fuel flows. The fuel filter of such a system is particularly prone to such blockage due to the minute openings required for effective filtration. Such "waxing" is especially prevalent at the present time due to the use of heavier grades of petroleum products in fuel oil to increase fuel production when crude oil is in short supply.
Engine stoppage due to "waxing" is particularly expensive in diesel powered vehicles, such as commercial trucks, which must be supplied with whatever fuel is available along the road, have their fuel tanks exposed to the ambient temperature, and must be serviced at considerable difficulty and expense, or retrieved for repair when the cause of engine stoppage is not obvious or cannot be corrected on the road.
In the past, it was possible to reduce "waxing" by mixing No. 1 fuel oil, a lighter grade, with No. 2; however, the present shortage of lighter grade petroleum products makes this prohibitively expensive even when No. 1 fuel oil is available. It is also well known to mix anti-waxing additives with diesel oil. However, such additives add to the cost of operation at best, must be carefully mixed, are relatively ineffective once wax has formed, and may not be effective if the fuel, as is frequently the case, contains water or is already mixed with additives. Stoppage of engines due to fuel filter waxing can, of course, be reduced by changing the fuel filter frequently. However, daily changes are sometimes required for commercial trucks and such frequent changes are relatively expensive, are likely to be neglected, and are not completely effective in cold weather or when heavy fuel is used.
Due to the above-described difficulties, it is well known to maintain diesel fuel at above approximately 60.degree. F. (15.6.degree. C.) where wax does not form in the fuel. It is, however, necessary to prevent the fuel temperature from rising substantially over 100.degree. F. (37.7.degree. C.) for proper lubrication and cooling of the fuel injectors of a diesel engine and to prevent the formation of harmful substances in the fuel. Prior art fuel-heating devices have, therefore, been provided with temperature-sensitive controls which actuate valves or the like to maintain fuel temperatures in the desired range. These controls are relatively expensive and have moving parts which are subject to wear and breakage. Such controls are particularly disadvantageous on diesel-powered vehicles which are subject to road shocks and high and low temperature extremes and where the controls cannot be attended to or replaced during operation.
The necessary heat for certain prior art fuel-heating devices has been obtained from engine coolant. Deriving heat from engine or transmission lubricant has been considered. Since these fluids must be conducted from and returned to the engine or transmission, the necessary plumbing is relatively involved, presents the danger of leaks, and where lubricants are used, of fire. Further, the additional cooling of coolants and lubricants when heat is transferred to the fuel may result in undesirably low coolant or lubricant temperatures. Since these heat sources are at a relatively low temperature, the required heat-transfer surface for effective heating in cold weather is relatively large. However, controls are still needed to prevent overheating of the fuel when the coolant or lubricant temperature is high and/or when the weather is hot. The use of coolants or lubricants for heating is also disadvantageous since their temperatures remain low for some time after starting and during idle operation. As a result, the danger of waxing is still present during these conditions.
The use of exhaust heat to heat internal combustion engine fuel, including diesel oil to prevent waxing, is well known. The use of heat from the engine exhaust eliminates a number of the above-described problems, such as the need for extra plumbing. However, the relatively high exhaust temperature, having a range of 250.degree. to 900.degree. F. (121.degree. to 482.degree. C.), which is beneficial in certain respects, easily produces excessive fuel temperatures. The exhaust temperature also changes substantially instantaneously when load on the engine changes. As a result, the prior art devices using exhaust heat for diesel fuel heating require either sensitive, rapid-acting, and therefore, expensive and relatively fragile control elements or other complications to prevent occasionally excessively high fuel temperatures.
Prior art devices for heating fuel having employed a coil of metallic tubing wound tightly about a metallic conduit through which the exhaust gases flow from an internal combustion engine. This provides a relatively simple, economical, and rugged fuel heating device. However, direct control of the heating coil temperature is not possible when the coil engages the conduit. In fact, it has been found that excessive fuel temperatures result even when such a coil is spaced from such a conduit when the coil and conduit are surrounded by a gaseous medium, such as air. As a result, temperature control of the coil is still required with this spaced arrangement when the purpose of the heater is to heat diesel fuel to prevent cold weather waxing. The broad essence of the present invention resides in the discovery that if such a coil is spaced from such a conduit and both immersed in a heat-conductive fluid, excellent fuel heating can be quickly and dependably achieved and the risk of overheating even under extreme conditions avoided.